Actually, out of the four most populous states, first cousin marriages are allowed in California, New York and Florida while Texas does not allow it. So there is no real pattern based on either religious or conservative views. Utah, probably the most religious state, allows it only if one of the couple is proven infertile.

Those are places where it's legal, but in the population centers of the first, and at least the largest of the second, at least among the dominant culture, it's very, very taboo, really less for any prudish or genetic reason than because it's associated with the people brasca mentions. Admit to sex with a cousin in those places, probably the first response you get won't be "gross" or "scum," it'll be someone humming Dueling Banjos.

Well let's just remember that even as late as the 1970's it was taboo to marry someone outside your race, even in California where I grew up. Even the most progressive populations in this country are not nearly as accepting as they like to think they are.